In recent years it has become obvious that the generation of greenhouse gases leads to global warming and that further increase in greenhouse gas production will accelerate global warming. CO2 (carbon dioxide) is identified as a main greenhouse gas and NOx is believed to significantly contribute to the greenhouse effect as an indirect greenhouse gas by producing ozone in the troposphere. CCS (carbon capture and storage) and reduction of NOx emissions are considered potential major means to reduce and to control global warming.
Reduction of NOx emissions can be achieved either by catalytic cleaning of the flue gases or by reduction of the NOx production during combustion.
There has been a continuous strive for higher hot gas temperatures to increase power plant efficiencies. However, NOx emissions increase with higher combustion temperature. To counter this effect flue gas recirculation has been suggested.
CCS can be defined, for example, as the process of CO2 capture, compression, transport and storage. Capture can be defined, for example, as a process in which CO2 is removed either from the flue gases after combustion of a carbon based fuel or the removal of and processing of carbon before combustion. Regeneration of any absorbents, adsorbents or other means to remove CO2 from a flue gas or fuel gas flow is considered to be part of the capture process.
Backend CO2 capture or post combustion capture is a commercially promising technology for fossil fuelled power plants including CCPP (combined cycle power plants). In post-combustion capture the CO2 is removed from a flue gas. The remaining flue gas is released to the atmosphere and the CO2 is compressed for transportation, and storage. There are several technologies known to remove CO2 from a flue gas such as absorption, adsorption, membrane separation, and cryogenic separation. Power plants with post-combustion capture are the subject of this disclosure.
All known technologies for CO2 capture specify the use of relatively large amounts of energy. Due to the relatively low CO2 concentration of the flue gases of a conventional CCPP, such as 4% for example, the CO2 capture system (also called CO2 capture plant or CO2 capture equipment) for a conventional CCPP can be more costly and energy consuming per kg of captured CO2 than the ones for other types of fossil power plants, such as coal fired plants, which have a relatively higher CO2 concentration.
The CO2 concentration in the CCPP flue gas can depend on the fuel composition, gas turbine type and load and can vary substantially depending on the operating conditions of the gas turbine. This variation in the CO2 concentration can be detrimental to the performance, efficiency, and operatability of the CO2 capture system.
To increase the CO2 concentration in the flue gases of a CCPP two main concepts are known. One concept can involve the recirculation of flue gases as for example described by O. Bolland and S. Saether in “NEW CONCEPTS FOR NATURAL GAS FIRED POWER PLANTS WHICH SIMPLIFY THE RECOVERY OF CARBON DIOXIDE” (Energy Conyers. Mgmt Vol. 33, No. 5-8, pp. 467-475, 1992). Another concenpt can involve the sequential arrangement of plants, where the flue gas of a first CCPP is cooled down and used as inlet gas for a second CCPP to obtain a flue gas with increased CO2 in the flue gas of the second CCPP. Such an arrangement is for example described in US20080060346. These methods can reduce the total amount of flue gas discharged to ambient surrounding and can increase the CO2 concentration, and thereby reduce the specified flow capacity of an absorber, the power consumption of the capture system, the capital expenditure for the capture system, and increase the CO2 capture system's efficiency. However, flue gas recirculation reduces the oxygen content in the inlet gases of the gas turbine and affects combustion. Besides positive effects on NOx emission the reduced oxygen content can lead to an incomplete unstable combustion and result in high CO emissions, which is highly undesirable.